Sampling of electrical signals is carried out for a large variety of reasons. One application is in the study of chemical reducing or oxidizing reactions in photographic solutions in order to determine the redox potentials, which are proportional to the concentration of various components, or species, therein.
An electronic sampling circuit is known using square-wave voltametry, in which current drawn by an electrode immersed in an electrolyte is sampled at the top and bottom of each pulse of a voltage square wave signal applied as a linear sweep across the electrolyte. The voltage square wave is progressively increased by predefined increments and the successive currents from the electrode are analyzed. FIG. 1 of the accompanying drawings shows the waveform of the applied voltage signal. FIG. 2 of the drawings shows a block diagram of the known sampling circuit. Referring to FIGS. 1 and 2, the voltage square wave represented in FIG. 1 is applied to a three-electrode geometry potentiostat system 2 of FIG. 2. The potentiostat 2 uses an amplifier 4 to amplify the applied signal before it reaches a counter electrode immersed in an electrolyte 8 in test cell 10. Current flow in the liquid 8 is collected at a working electrode 12 and conducted to an input terminal 14 of a current-to-voltage converter/amplifier 16 to provide an output signal on path 18 that is a measure of the electrolyte current, in square-wave form. The terminal 14 of the converter/amplifier 16 is maintained at virtual earth potential, and a third electrode, reference electrode 20, immersed in the electrolyte 8 between the counter and working electrodes 6,12, forms a feedback loop via an amplifier 22 to another input terminal of the amplifier 4. The measured voltage signal on the path 18 is fed to a sample-and-hold circuit 24. Initially, sampling takes place at Sample Point 1, the peak of the first square wave pulse of the measured current as shown in FIG. 1. This value is converted to a digital code by an analogue-to-digital converter (AID) 26 prior to being stored in a microprocessor 28. The input signal on path 18 is then sampled by the circuit 24 at Sample Point 2, the trough of the first square wave pulse of the measured current as shown in FIG. 1, converted to a digital signal by the converter 26 and stored in the microprocessor 28. Once both of these values are stored, they are subtracted using software in the microprocessor 28 to produce a difference signal that is representative of the current flowing in the electrolyte in response to the applied square-wave voltage. This measurement is then repeated for each pulse of the square wave on the ramp for, say, 1000 readings, or more.